Side-Chain Radicals

Since pure PP does not Incorporate chromophorlc groups, it is clear that photolnltlatlon of radical degradation processes must Involve chromophorlc impurities. There has been a great deal of discussion of this in the past and hydroperoxides or carbonyl structures formed by oxidation of the parent polymer and transition metal residues from the polymerization catalyst seem to be the most likely candidates. It is not appropriate to discuss this aspect in the present paper, suffice it to say that the association of methane with photolnltlatlon, but not thermal Initiation, suggests that photolnltlatlon Involves C-CH3 bond scission to form chain side radicals in contrast to thermal Initiation which involves scission of the C-C bond In the main chains. 

The Initial formation of chain side radicals rather than chain terminal radicals as In the thermal degradation also allows a reasonable explanation of some of the other features of the photoreaction. Thus the chain side radicals should be expected to undergo a number of competing reactions of the type shown. 

Scission of these chain side radicals can result In radicals terminated by sequences of methylene groups which should be capable of liberating a proportion of monomeric ethylene by depoly-merlzatlon (reactions 6 and 8). This Is very much less probable if radicals are initially formed by main chain scission as in the thermal reaction. 

Irradiation of PMMA at 20 C results In chain scission and the production of terminally unsaturated polymer molecules, but no large production of volatile products. At 150°C the increased mobility of the polymer molecules allows the primarily formed radicals to depropagate and monomer production is quantitative. Irradiation of PP results in chain side radicals which combine to form cross links at 20 C, but undergo scission to form terminally unsaturated molecules at ISO C. VHien blends are irradiated at these temperatures, there appears to be no significant interaction of the radicals formed in the two phases because there is no evidence of block or graft formation. Thus the two constituents appear to decompose In Isolation from one another. 

Only a small fraction of reactant is iavolved ia this step. When naphthenes are iavolved, diradicals are produced. Eor aromatics with side chains, H radicals are produced. 

Although peroxide initiated reactions can produce preparative yields of organomercury salts, a mixture of products was usually obtained due to chain mechanism/radical displacement competition and to the prevalence of side reactions [Eqs. (24), (25), and (109)]. An alternative to the... 

Table 4.1 weighs the positional selectivity of the side-chain cation-radical acetoxylation against the side-chain pure radical bromination. 

The chain ion-radical mechanism of ter Meer reaction has been supported by a thorough kinetic analysis. The reaction is well-described by a standard equation of chain-radical processes (with square-law chain termination) (Shugalei et al. 1981). This mechanism also explains the nature of side products—aldehydes (see steps 13 and 14) as well as vicinal dinitroethylenes. Scheme 4.37 explains formation of vic-dinitroethylenes. 

The benzimidazole group of anthelminthics is derived from the simple benzimidazole nucleus and includes the thiabendazole analogues and the benzimidazole carbamates. Substitution of side chains and radicals on the benzimidazole nucleus gives rise to the individual members of this group (Fig. 4.1). 

For proteins and peptides having sulfydryl or mercaptal side chains, free radical disproportionation reactions such as the ones reported by... 

For the initiation by azo initiators only the dependence kp / kt0 5=f ([M]) has to be considered in a kinetic model [10]. Accordingly, an initiator exponent of 0.5 and a monomer exponent of 2 are valid. By adding amine the decomposition velocity of APS is increased by an orders of magnitude. The chain side reactions with the monomer and termination by chlorine atoms are then significantly suppressed which results in a monomer exponent of 2 and higher molar masses of the homopolymer [11]. The kinetics of 2.3 order in monomer and 0.47 order in initiation [59], explained by partial cyclization and termination of cyclized radicals, could not be confirmed. 

There may be common themes in the role of protein-coenzyme contacts in these B -dependent enzymatic processes. In particular, these contacts could alter the relative stability of the Co(III)—R, Co(II), and Co(I) states to enhance reactivity. For coenzyme B 12-dependent enzymes, the deoxyadenosyl radical generates a substrate-derived radical, either directly or via a radical chain mechanism through the intermediacy of a protein-side-chain-based radical, such as S of cysteine or O of tyrosine. This protein-bound substrate-derived radical then undergoes rearrangement, possibly assisted by protein contacts. Thus, cofactor-protein contacts are probably very important in the activation of the Co—C bond, in altering the Co redox potentials, and in assisting in the rearrangements. 

So far, there have been only few reports about the synthesis of amphipolar polymer brushes, i.e. with amphiphilic block copolymer side chains. Gna-nou et al. [115] first reported the ROMP of norbornenoyl-endfunctionalized polystyrene-f -poly(ethylene oxide) macromonomers. Due to the low degree of polymerization, the polymacromonomer adopted a star-like rather than a cylindrical shape. Schmidt et al. [123] synthesized amphipolar cylindrical brushes with poly(2-vinylpyridine)-block-polystyrene side chains via radical polymerization of the corresponding block macromonomer. A similar polymer brush with poly(a-methylstyrene)-Wocfc-poly(2-vinylpyridine) side chains was also synthesized by Ishizu et al. via radical polymerization [124]. Using the grafting from approach, Muller et al. [121, 125] synthesized... 

Side-chain aromatic radicals of the type Ph—CR—OH cannot be produced efficiently by reaction of OH with the alcohol because OH tends to add to the aromatic ring more efficiently. They are, therefore, produced by addition of an electron to the carbonyl compound. The pRwalues for the acetophenone and benzophenone ketyl radicals are 9-5 and they are affected by substituents on the aromatic ring as well as by those directly at the radical site. ... 

SCHEME 14.1 The general stmcture of an acrylic polymer and the estahhshed photodegradation mechanism via Norrish I a-cleavage of the carhonyl side chain, leading to main-chain polymeric radical a and oxo-acyl radical b. The secondary P-scission rearrangement reaction leading to the propagating radical c is also shown. 

All these spectra were acquired at elevated temperatures ( 100°C), that is, where the observation of fast motion spectra is expected. In Fig. 14.4A, the TREPR spectrum of the main-chain polymer radical from photolysis of /-PMMA is repeated from the bottom left side of Fig. 14.2, as it is the starting point for comparisons of spectral features such as hnewidths and hyperfine coupling constants. The nomenclature used throughout this section is derived using the notations indicated in Scheme 14.1 and Chart 14.1. For example, a main-chain radical from PMMA will be denoted la, whereas the oxo-acyl radical from PFOMA will be designated as radical 6b, and so on. For all radicals simulated, the parameters used are listed in Table 14.1. 

The TREPR experiments and simulations described here have provided an enormous amount of structural and dynamic information about a class of free radicals that were not reported in the hterature prior to our first paper on this topic in 2000. Magnetic parameters for many main-chain acrylic radicals have been established, and interesting solvent effects have been observed such as spin relaxation rates and the novel pH dependence of the polyacid radical spectra. It is fair to conclude from these studies that the photodegradation mechanism of acrylic polymers is general, proceeding through Norrish 1 a-cleavage of the ester (or acid) side chain. Recently, model systems have... 

Lipid radical transfer has been demonstrated for trp, arg, his, and lys (99, 383, 384), all of which have reactive N groups on their side chains, and radical decomposition products from these amino acids have been identified (381, 382, 390). Tyrosine and methionine degradation by oxidizing lipids has also been demonstrated (390), but the intermediate radicals in the reaction may be too unstable for detection. Lipid radical adducts to amino acids are important flavor precursors (340) and also may play critical roles in pathological processes in vivo (186, 388). 

The predominance of nuclear methoxylation over competing alkyl side-chain oxidation processes for these systems was largely attributed to an intramolecular attack by the appended side-chain alkoxy radical on the anodically generated aromatic radical cation in the usual EEQCp mechanism [99]. A more recent mechanistic study of anodic oxidation of chiral alkoxynaphthalene derivative (LXXXIX) sheds some additional light on this process [Eq. (46)] [100]. The isolation of a 2 1 mixture of enantiomeric methoxylated products indicated that intramolecular attack by the appended alkoxy radical on the aromatic radical cation (path B) was disfavored at —7S°C (a nearly 1 1 mixture of enantiomers was obtained at 25 C), since the intermediate cyclic ketal resulting from such a process would be a meso form that would lead to a racemic mixture of methoxylated products. Note that the chirality of the appended hydroxyether side-chain disappears upon cycliza-tion to the acetal. 

Substitution in the side chain. Free-radical halogenation. Discussed in Secs. 12.12 12.14. 

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